1. Field of the Invention
The present invention relates to a flux cleaning process suitable for processing semiconductor devices, and to a method of manufacturing the semiconductor devices.
2. Related Art
Along with a recent development in a high density packaging technology for packaging electronics components and parts, a further compactness in a semiconductor package is demanded. Now, as such an example of semiconductor packaging for providing a compacter design of electronic devices, a flip-chip connection process is drawing attention and becoming to be an indispensable process therefor, whereby a solder bump which is formed on an electrode pad on a semiconductor chip is electrically and physically connected with an electrode terminal provided on a packaging printed circuit board.
This method of forming the solder bump can be divided roughly into two approaches: one to form solder bumps in batch on a wafer; the other to form solder bumps respectively on an individual semiconductor chip after it was diced from the wafer. However, it is preferable to form solder bumps in batch on the wafer from the viewpoint of economy of a production cost.
As an example of forming solder bumps in batch in a stage of the wafer for reducing the production cost significantly, there is such a process of forming solder bumps whereby, by using a high precision printing method using a solder paste and a screen mask, solder is transferred onto a semiconductor chip electrode to form a solder bump.
Now, with reference to FIGS. 8A through 10B, a method of forming solder bumps by printing will be described by way of example of a solder paste 32 printed on a semiconductor wafer 1. FIGS. 8A to 10B show cross-sections of a screen mask 41 and a wafer 44 in each step of the solder printing method.
With reference to FIG. 8A, at first, a bottom surface 42 of the screen mask 41 having perforation 43 formed in a predetermined pattern is placed on a wafer 44 in face-to-face contact. Then, a solder paste 32 is supplied to an upper surface 37 of the screen mask 41. In the next step of FIG. 8B, a squeegee 30 with its tip end contacting the upper surface 37 of the screen mask is moved in a direction of an arrow under application of a downward pressure, so as to roll and fill the solder paste 32 into the perforation 43 of the screen mask 41.
Then, as illustrated in FIG. 9A, the screen mask 41 is lifted off from the wafer 44. As a result, as shown in FIG. 9B, the solder paste 32 filled in the perforation holes 43 is transferred onto the wafer 44, thereby depositing the solder paste 32 on an electrode pad 36 formed on the wafer 44 in a predetermined pattern.
Subsequently, as illustrated in FIG. 10A, the wafer 44 is wholly heated with a heating system such as a heater 45 or the like. A rosin group flux contained in the solder paste 32, when heated by the heater 45, is activated to effectuate removal of an oxide on the electrode pad 36 by reduction reaction so as to provide a clean metal surface. Then, a powder solder contained in the solder paste 32 is melted to form a round swell by a surface tension, which then wets and spreads only on the surface of the electrode pad 36 which is cleaned by the flux, then to form a bump electrode 35. This heating treatment is generally referred to as a xe2x80x9cwet backxe2x80x9d.
As illustrated in FIG. 10B, compositions of the flux contained in the solder paste 32 which spill out while heating adhere, as a flux residue 34, to a surface of the solder bump electrode 35 and a peripheral area in the vicinity of a bordering region between the solder bump electrode 35 and the wafer 44. Although the flux residue 34 adhered to the surface of the solder bump electrode 35 can be removed by a normal flux cleaning process using, for example, a hydrocarbonic group cleaning agent, the other flux residue 35 adhered to the peripheral area in the vicinity of the bordering region between the solder bump electrode 35 and the wafer 44 cannot be removed.
Then, as illustrated in FIG. 11, a semiconductor chip 38 diced from the wafer 44 into an individual semiconductor chip is soldered onto an electrode portion 40 of a printed circuit board 39 via a bump electrode 35. At the time of soldering, because the flux residue 34 remains on the surface of the solder bump electrode 35, unless cleaning thereof is done sufficiently, the flux residue 34 is included between the surface of the printed circuit board electrode 40 and the surface of the bump electrode 35, and is left as inclusion in a portion of the solder bump connection, thereby deteriorating an adhesion strength and reliability, and further causing an insufficient wettability portion 46.
Generally, in order to increase adhesion strength in the solder bump connection portion, a resin 31 is filled in a gap between the semiconductor chip 38 and the printed circuit board 39 to seal therebetween. However, if a sticky flux residue 34 is left in the vicinity of the surface of the semiconductor chip 38, a filling performance of the sealing resin 31 is affected to deteriorate the adhesion strength between the semiconductor chip 38 and the sealing resin 31, thus degrading the reliability of the device. For this reason, it is especially important and indispensable completely to remove the flux residue 34 from the periphery of the solder bump electrode 35 prior to mounting the semiconductor chip on the printed circuit board 39.
As a rosin group flux cleaning agent, solvents such as Freon (chlorofluorocarbon) or trichlene (trichloroethylene) cleaning agents have been used heretofore, however, as they may involve environmental problems such as destruction of the ozone layer, global pollution or the like, hydrocarbon group and terpene group cleaning agents are now being used instead of the Freon or trichlene cleaning agents. By the way, when using these cleaning agents they are heated. Although a cleaning performance improves at higher temperatures, in consideration of various factors such as an increasing amount of evaporation at higher temperatures (in terms of economy), flammability, safety and the like, they cannot be used at temperatures higher than 80xc2x0 C. or so.
If these flux residues are tried to be removed by cleaning using these cleaning agents, however, these flux residues cannot be removed completely permitting some of the flux residues to remain adhering to the bordering and adjacent area between the solder bump electrode and the semiconductor chip. Normally, a thixotropic substance is contained in the solder paste for the purpose of securing its printed pattern to be retained, wherein an amido-fatty acid therein which is widely used because of its excellent performance cannot be removed easily with the normal cleaning agents for use in the normal flux cleaning. Although the flux residue may be removed by mechanically rubbing the wafer or by applying a ultrasonic cavitation (vibration) thereto, it is not permitted to give such a mechanical shock or damage to the semiconductor chip having bumps attached thereto.
The present invention has been contemplated to solve the above-mentioned problems associated with the related art, and to provide a novel method capable of efficiently cleaning flux residue which is produced in process of fabricating semiconductor devices, and a method of manufacturing semiconductor devices including this novel cleaning method.
Namely, the present invention is directed to a method of flux cleaning for cleaning a solder electrode portion which is formed using the flux, comprising: a step of coating a flux on the solder electrode portion, a step of a heat treatment thereof, and a step of the cleaning. Further, the present invention is directed to a method of manufacturing a semiconductor apparatus, comprising: a step of mounting a semiconductor chip on a printed circuit board using the solder electrode portion after the flux cleaning.
According to the flux cleaning process according to the present invention, comprising the steps of executing preliminary cleaning process using the flux, then applying the normal cleaning process using such as hydrocarbonic group or terpene group cleaning agents, is applied to a semiconductor wafer or the like as a pretreatment step prior to subjecting to the normal flux cleaning process therefor, a complete flux residue-free cleaning of the wafer can be attained. This flux cleaning process can be applied widely irrespective of the types of substrates on which the solder paste printing is to be applied, therefore, this is also applicable to cleaning of the flux on the solder bump electrode formed on the semiconductor wafer using the solder paste printing process. Further, this process can be applied to the process of forming solder bumps using a metal ball instead of the solder paste, or to a process of forming solder bumps on the side of a printed circuit board.